Conventional articles of athletic footwear have included two primary elements, namely an upper member and a sole structure. The upper member provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper member may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure generally is secured to a lower portion of the upper member and generally is positioned between the foot and the ground. In addition to attenuating ground or other contact surface reaction forces, the sole structure may provide traction and control foot motions, such as pronation or suppination. Accordingly, the upper member and sole structure may operate cooperatively to provide a comfortable structure that is suited for a variety of ambulatory activities, such as walking, running or playing basketball.
A conventional athletic shoe includes an outsole, a midsole, and an upper. Such a shoe is typically designed to reduce the shock felt by the wearer during foot strike(s). Such reduction in shock may provide comfort and reduce the likelihood of injury to the wearer. Unstable shoes may cause short- or long-term injury due to the excessive motion at the joints brought on by unstable materials and designs.
Cushioning in most athletic shoes is supplied through a foam midsole made from ethylene vinyl acetate (EVA) or polyurethane (PU). These materials are relatively inexpensive, easily molded, and provide ample cushioning when they are new. Other shoes have used gas-filled and liquid-filled bladders to provide the required cushioning. Both of these shoe constructions provide adequate cushioning when they are new. Fluid-filled bladders continue to provide like-new cushioning for the life of the shoe, assuming that the fluid remains encapsulated in the shoe. In contrast, shoe midsoles made from foams provide adequate cushioning when they are new, but quickly lose some of their cushioning ability when the open cellular structure inside the foam suffer catastrophic failure from the application of vertical and/or shear forces. EVA foams have compression (compaction) set rates of greater than 50%. This means that the ability to provide cushioning is reduced by at least 50% due to compaction of the cushioning material. In contrast to EVA, PU generally has better compression set. However, the use of PU increases the weight of the shoe compared to the use of EVA.
In addition to cushioning, a shoe should also supply support and stability. Generally, as the materials used under foot become softer, the support and stability decrease. Harder/firmer materials lend the most support and stability. Since harder/firmer materials decrease the amount of available cushioning, providing adequate cushioning without detracting from support and stability may be a challenge that requires attention to detail with respect to material choices and design.